Satellite based positioning systems operate by utilizing constellations of satellites which transmit to earth continuous direct sequence spread spectrum signals. Receivers within receiving range of these satellites intercept these signals which carry data (navigation messages) modulated onto a spread spectrum carrier. This data provides the precise time of transmission at certain instants in the signal along with orbital parameters (e.g., precise ephemeris data and less precise almanac data in the case of GPS) for the satellites themselves. By estimating the time of flight of the signal from each of four satellites to the receiver and computing the position of the satellites at the times of transmission corresponding to the estimated times of flight it is possible to determine the precise location of the receiver's antenna.
In a conventional SPS receiver, the process by which this is done involves estimating pseudoranges of at least 4 satellites and then computing from these the precise location and clock error of the receiver. Each pseudorange is computed as the time of flight from one satellite to the receiver multiplied by the speed of light and is thus an estimate of the distance or ‘range’ between the satellite and the receiver. The time of flight is estimated as the difference between the time of transmission determined from the navigation message and the time of receipt as determined using a clock in the receiver. Since the receiver's clock will inevitably have a different present time when compared to the clock of the satellites, the four range computations will have a common error. The common error is the error in the receiver's clock multiplied by the speed of light.
By using at least 4 satellites it is possible to solve a set of equations to determine both the receiver clock error and the location of the antenna. If only 3 measurements are available it is still possible to determine the location and clock error provided at least one of the receiver's coordinates is already known. Often, this situation can be approximated by estimating the altitude of the antenna.
The signals from the satellites consist of a carrier signal which is biphase modulated by a pseudo-random binary spreading code at a relatively high “chipping” rate (e.g., 1.023 MHz) and then biphase modulated by the binary navigation message at a low data rate (e.g., 50 Hz). The carrier to noise ratio is typically very low (e.g., 31 dBHz to 51 dBHz) at the earth's surface for a receiver with unobstructed line of sight to the satellite from its antenna. However, it is sufficient to permit the signals to be detected, acquired and tracked using conventional phase-locked loop and delay-locked loop techniques and for the data to be extracted.
The process of tracking the code of a signal in a conventional SPS receiver involves the use of a hardware code generator and signal mixer. When the locally generated code is exactly aligned with that of the incoming signal, the output from the mixer contains no code modulation at all. Hence the bandwidth of the signal is much less and it can be filtered to greatly increase the signal to noise ratio. This is usually done using a decimation filter such that the correlator output sampling rate is much lower than the input sampling rate (e.g., 1 kHz at the output compared to 1.3 MHz at the input).
Also, in the case of GPS, the precise time of transmission of this signal corresponding to any given instant at the receiver can be determined by latching the state of the code generator to get the code phase and by counting the code epochs within each bit of the data and by counting the bits within each word of the navigation message and by counting the words within each subframe of the message and by extracting and decoding the times of transmission corresponding to the subframe boundaries. A similar scheme can be used for any SPS.
However, traditional SPS receivers can suffer from troublesome lapses in position identification in the presence of weakened transmission signals. When the direct line of sight between the antenna and the satellites is obstructed, signals may be severely attenuated when they reach the antenna. Conventional techniques can not be used to detect, acquire and track these signals. Moreover, under these circumstances even if the signal could be detected, the carrier-to-noise ratio of a GPS signal, for example, may be as low as or lower than 24 dBHz and as such it is not possible to extract the data from the signals.
Prior art devices have attempted to minimize or overcome these shortcomings through the use of aiding information. In such schemes, additional information is externally supplied to the SPS receivers through various secondary transmission sources to balance the shortfall of information resulting from the attenuated signals. Examples of such devices are taught in the patents to Taylor et al. (U.S. Pat. No. 4,445,118) (aided by satellite almanac data); Lau (U.S. Pat. No. 5,418,538) (aided by differential satellite positioning information and ephemerides); Krasner (U.S. Pat. No. 5,663,734) (aided by transmission of Doppler frequency shifts); Krasner (U.S. Pat. No. 5,781,156) (aided by transmission of Doppler frequency shifts); Krasner (U.S. Pat. No. 5,874,914) (aided by Doppler, initialization and pseudorange data) Krasner (U.S. Pat. No. 5,841,396) (aided by satellite almanac data); Loomis, et al. (U.S. Pat. No. 5,917,444) (aided by selected satellite ephemerides, almanac, ionosphere, time, pseudorange corrections, satellite index and/or code phase attributes); Krasner (U.S. Pat. No. 5,945,944) (aided by timing data); Krasner (U.S. Pat. No. 6,016,119) (aided by retransmission of data from satellite signal)
However, aiding information requires additional transmission capabilities. For example, aiding information may be sent to the SPS receiver using additional satellite transmitters or wireless telephone systems. As such, it is a significant advantage to reduce the quantum of aiding information supplied to limit the use of such additional resources. For example, when the voice path of a wireless communication network is being used to communicate the aiding information, the voice communication will be interrupted by the aiding message. The aiding messages must therefore be as short as possible in order to limit the voice interruptions to tolerable durations and frequencies. Also, no matter how the aiding data is communicated, its communication will delay the operation of the receiver. In many applications the location data is needed promptly and therefore any delay must be minimized.